onViewFinderTap method
Implementation
void onViewFinderTap(TapDownDetails details, BoxConstraints constraints) {
if (controller == null) return;
final CameraController cameraController = controller!;
final offset = Offset(details.localPosition.dx / constraints.maxWidth,
details.localPosition.dy / constraints.maxHeight);
cameraController.setExposurePoint(offset);
cameraController.setFocusPoint(offset);
}